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<div class="titlepage"><div><div><h4 class="title">
<a name="boost_asio.overview.composition.futures"></a><a class="link" href="futures.html" title="Futures">Futures</a>
</h4></div></div></div>
<p>
          The <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">use_future</span></code> completion token provides
          first-class support for returning a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">future</span></code>
          from an asynchronous operation's initiating function.
        </p>
<p>
          To use <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">use_future</span></code>, pass it to an asynchronous
          operation instead of a completion handler. For example:
        </p>
<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">future</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span><span class="special">&gt;</span> <span class="identifier">length</span> <span class="special">=</span>
  <span class="identifier">my_socket</span><span class="special">.</span><span class="identifier">async_read_some</span><span class="special">(</span><span class="identifier">my_buffer</span><span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">use_future</span><span class="special">);</span>
</pre>
<p>
          Where a completion signature has the form:
        </p>
<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">handler</span><span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">system</span><span class="special">::</span><span class="identifier">error_code</span> <span class="identifier">ec</span><span class="special">,</span> <span class="identifier">result_type</span> <span class="identifier">result</span><span class="special">);</span>
</pre>
<p>
          the initiating function returns a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">future</span></code>
          templated on <code class="computeroutput"><span class="identifier">result_type</span></code>.
          In the above example, this is <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span></code>.
          If the asynchronous operation fails, the <code class="computeroutput"><span class="identifier">error_code</span></code>
          is converted into a <code class="computeroutput"><span class="identifier">system_error</span></code>
          exception and passed back to the caller through the future.
        </p>
<p>
          Where a completion signature has the form:
        </p>
<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">handler</span><span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">system</span><span class="special">::</span><span class="identifier">error_code</span> <span class="identifier">ec</span><span class="special">);</span>
</pre>
<p>
          the initiating function returns <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">future</span><span class="special">&lt;</span><span class="keyword">void</span><span class="special">&gt;</span></code>. As above, an error is passed back
          in the future as a <code class="computeroutput"><span class="identifier">system_error</span></code>
          exception.
        </p>
<h6>
<a name="boost_asio.overview.composition.futures.h0"></a>
          <span class="phrase"><a name="boost_asio.overview.composition.futures.see_also"></a></span><a class="link" href="futures.html#boost_asio.overview.composition.futures.see_also">See
          Also</a>
        </h6>
<p>
          <a class="link" href="../../reference/use_future.html" title="use_future">use_future</a>, <a class="link" href="../../reference/use_future_t.html" title="use_future_t">use_future_t</a>, <a class="link" href="../../examples/cpp11_examples.html#boost_asio.examples.cpp11_examples.futures">Futures example (C++11)</a>.
        </p>
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